Cracking the units open revealed a somewhat ugly bias battery assembly, comprised of two 625 mercury batteries bound together. This is only switched in at 1000 gain, and so I opted to remove it with the intent to replace in event of malfunction. My applications are confined to the audible spectrum of sound (as opposed to full 5Hz-2MHz claimed) so a little non-linearity may actually be welcome.

There's really only one way to find out.

Original circuit, gutshot topside.

Original circuit, gutshot from below. My intent is to replace the electrolytics and the selenium, and then give it a listen.

Modern parts make for a roomier experience.

A 100pf mica domino cap (visible on the original unit) has been installed on the wafer switch.

I've been going the budget route on electrolytic capacitors, building ugly little clusters costs less than a quarter of repop multisection can caps. These are not museum collection pieces, these are supposed to be used.

This was an easy fix that entailed shortening the power cable about a foot from the inside.

General purpose gutshot.

The earlier, original screw is quite evident compared to the newer screw, both of which exhibit a level of quality I don't see in a lot of current hardware.

So, after working one unit over I was slowly bringing it up on a variac and started to read voltage swings on output, one pulse every two seconds or so. Slow enough to confuse my multimeter on both AC & DC settings. The scope displayed a pulse that jumped toward the top of the scale, which indicated a frightful signal to be feeding gear downstream.

Plugging this 75 ohm terminator from my video junk into the input happens to snub the oscillation. It took me a little while to connect this particular set of dots.. after discovery of the motorboating, I shelved this for a while to clear my head. Then I observed that it didn't oscillate at 10, various resistors strapped on the output, loading the input with a signal source and listening to output, generally just fooling around with it brought about this observation.

So, the plan is to terminate the input with the largest resistor I can to prohibit runaway, which, I might add, only affects this unit (before and after recap). I dug into this one first because it was operationally impaired, so I could have a functioning unit to A/B while troubleshooting, should the need arise.

All in all, I would not describe this as a high fidelity preamp, it does however go deeply enough into the realm of character to justify having around. Plus, it's pleasing to the eye.